The present invention relates to an improved process for making hermetically sealed filter units. In particular, the present invention relates to a process for bonding a porous filter element to a thermoplastic skirt to insure an integral seal. The present invention also relates generally to hermetically sealed filter units and, more specifically, to such filters which are used to remove bacteria and other minute contaminants from a fluid stream. In addition, the present invention relates to injection molded thermoplastic filter housings which include a novel arrangement of structural features to improve both the quality of the manufacturing process and the integrity of the seal.
Molded plastic filters and filter units are well known and enjoy a wide use and general acceptance in a variety of industries. The function of such units is to remove contaminants from liquid or gaseous materials which flow therethrough. These units are particularly important in the medical industry where they are used to filter bacteria, leukocytes, clots, particles, gases and other contaminants from solutions before being introduced into the human blood stream. In such critical applications, it is imperative that the fluid not bypass the filtering elements. It is necessary, therefore, to provide an undamaged leak-proof filter unit to prevent leakage around the filter element and to prevent possible bacteria contamination from outside the assembly.
Often, the filter units used to filter intravenous solutions are made up of a two-part housing with an internally disposed filter sandwiched between the housing members. Typically, the manufacturing process for thermoplastic filter units entails first the injection molding of the housing members. Once the housing members have been molded, the filter element is usually placed on the outlet housing member over the outlet opening. The inlet housing element is then positioned over the outlet housing member sandwiching the filter element between the two housing members. The edge of the filter element remains exposed at the joint between the housing members. The inlet housing member, outlet housing member and filter element are then sealed together by any of a variety of methods including, pressure clamping, heat sealing, ultrasonic welding, or the use of a thermoplastic overmold band. In particular, the use of an overmold band offers the advantage of adding significant structural strength to the filter unit.
Two U.S. patents generally describe the process of sealing a filter with a thermoplastic overmold band. U.S. Pat. No. 4,113,627, issued to Leason on Sep. 12, 1978, describes a process for the injection molding of a thermoplastic overmold band over two thermoplastic housing members having an exposed filter element sandwiched therebetween. U.S. Pat. No. 4,414,172, issued to Leason on Nov. 8, 1983, describes a process for the injection molding of a thermoplastic overmold band over one housing member and an exposed filter element. Both patents are incorporated herein by reference.
It is especially critical in the medical industry to maintain a leak-proof seal between a filter element and the filter housing members. Thus it is imperative that the injection molding process placing the thermoplastic overmold band create a perfect seal around the entire periphery of the filter element and the filter housing members.
In the prior art process, the filter edge remains exposed between the housing members so that the thermoplastic overmold band forms an integral bond to the edge of the filter element as well as to the housing members. To ensure that a seal has formed over the entire edge of the filter element, the overmold cavity in the mold must be completely filled during the injection molding process. High temperatures and pressures are necessary to ensure a complete seal. However, sometimes the high pressure causes the thermoplastic melt to force itself between the filter element and the housing member. The thermoplastic melt thereby "jets" into the filter cavity or "jets" in between layers of a multi-layered filter element. The problem of "jetting" is particularly acute when using multiple cavity molds, where even higher temperatures and pressures are needed to assure that each filter in the mold is completely sealed by the overmold band. This "jetting" problem results in a defective filter unit. The excess "jetted" thermoplastic reduces the surface area of the filter element useful for filtering the fluid and may also block the fluid passageways in the interior of the filter cavity.
Adjusting the temperature and pressure of the injected thermoplastic does not always solve this problem. Lower temperature may prevent jetting but the surface of the thermoplastic melt may cool and form a skin that prevents it from infusing into the edge of the filter element, thereby failing to form a hermetic seal. Likewise, lower pressure may prevent jetting but at the same time may cause voids in the overmold cavity, thereby failing to make a continuous seal around the complete periphery of the filter element.
Pressure clamping during the overmold step may help solve this "jetting" problem, but it is not suggested for use in many types of filters. Pressure clamping may distort the filter media or actually damage the filter media at the clamping edges, thereby destroying the integrity of the filter media and allowing contaminants to pass through. Moreover, with certain types of thermoplastic resins used in the construction of the housing, pressure clamping may cause stress induced fractures in the filter housings.
Also, heat sealing, sonic welding, and related thermomechanical bonding methods may damage the filter medium. These sealing methods may be unreliable, especially when the filter unit or the filter element has an uneven, irregular or unsymmetrical shape.
The use of adhesives or solvent bonding has disadvantages in that another material is introduced that can lead to contamination on its own. Often the constituents of an adhesive or solvent system may be damaging to any microporous membrane used in the filter.
Other methods found in the prior art for sealing the peripheral edge of a filter element do not solve this particular "jetting" problem, and are otherwise disadvantageous because they introduce additional steps in the manufacturing process or increase the number and complexity of the parts used for assembling a filter unit. Also, some of these methods may use materials which are not compatible with the intended use of a filter unit for the medical industry or specifically for the use with intravenous solutions. In addition, some methods cannot be performed in conjunction with a thermoplastic injection molding process.
Therefore, a need exists for a process which will eliminate the problem of jetting found in the prior art methods of injection molding a thermoplastic overmold band to seal a filter element. Also, a need exists for a process which does not rely upon complicated injection molding control systems, complex parts and additional manufacturing steps to solve the "jetting" problem.